“At the moment the casualty numbers appear to be very low … the bombs appear to be smaller” than those detonated July 7.

Police also said an armed police unit had entered University College hospital. Press Association, the British news agency, said they arrived shortly after an injured person was carried in.

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Sky News TV reported that police were searching for a man with a blue shirt with wires protruding. In a memo to hospital staff, officers asked employees to look for a black or Asian male, 6 feet 2 inches tall, wearing a blue top with a hole in the back and wires protruding.

The explosions did not shut down the subway system, although three lines remained closed more than two hours later. Blair appealed to Londoners to stay where they are until they hear more from police.

“What we don’t want is lots of people going to railway stations,” he said.

One witness told The Associated Press he smelled something similar to an electrical fire.

Police in chemical protection suits were seen preparing to enter the Warren Street Underground station. Sky News reported that police said no chemical agents were involved in the explosions.

Explosions also were reported at the Shepherd’s Bush and Oval stations.

Emergency teams were sent to all three stations after the incidents, which began at 12:38 p.m. One witness told Sky TV that another subway passenger told him a backpack exploded at Warren Street and there were reports of smoke.

Stagecoach, the company which operates the stricken bus, said the driver heard a bang and went upstairs, where he found the windows blown out. The company said the bus was structurally intact and there were no reports of injuries.

Closed-circuit TV cameras on Hackney Road showed the No. 26 bus immobilized at a stop with its indicator lights flashing. The area around the bus had been cordoned off.

The incidents paralleled the blasts two weeks ago, which involved explosions at three Underground stations simultaneously ” quickly followed by a blast on a bus. Those bombings, during the morning rush hour, also occurred in the center of London, hitting the Underground railway from various directions.

Thursday’s incidents, however, were more geographically spread out.

London Ambulance said it was called to the Oval station at 12:38 p.m. and Warren Street at 12:45 p.m. The July 7 attacks began at 8:51 a.m.

“People were panicking. But very fortunately the train was only 15 seconds from the station,” witness Ivan McCracken told Sky news.

McCracken said another passenger at Warren Street claimed he had seen a backpack explode. The bombs which killed 56 people on board three underground trains and a bus in London on July 7 were carried in backpacks, police said.

McCracken said he smelled smoke and that people were panicking and coming into his carriage. He said he spoke to an Italian man who was comforting a woman after the evacuation.

“He said that a man was carrying a rucksack and the rucksack suddenly exploded. It was a minor explosion but enough to blow open the rucksack,” McCracken said.

“The man then made an exclamation as if something had gone wrong. At that point everyone rushed from the carriage.”

Services on the Victoria and Northern lines were suspended following reports of a number of incidents, London Underground said.

“I was in the carriage and we smelt smoke – it was like something was burning,” said Losiane Mohellavi, 35, who was evacuated at Warren Street.

“Everyone was panicked and people were screaming. We had to pull the alarm. I am still shaking,” Mohellavi said.

He told the AP he did not see smoke but rather smelled something similar to an electrical fire.

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Associated Press reporters Thin Lei Win and Kate Bouey in London contributed to this report.